<%@ page language="java" contentType="text/html; charset=UTF-8"
    pageEncoding="UTF-8"%>
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
<title>Insert title here</title>
<script type="text/javascript">
	function f1(){
		if(frm.username.value==""){
			alert("用户名");
		}
		
	}
</script>
</head>
<body>
	<form action="" name="frm">
		<input type="text" name="username">
		<input type="button" value="提价" onclick="f1()">
	</form>
</body>
</html>